1. Field of the Invention
The present invention relates to a liquid crystal display element that modulates and emits incident light and a liquid crystal display device which displays an image with the use of light modulated by such a liquid crystal display element.
This application claims the priority of the Japanese Patent Application No. 2003-392364 filed on Oct. 21, 2003, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
There are available various types of display devices such as projection displays (projector), various types of portable electronic devices and various of information processing terminals. Each of such devices uses a liquid crystal display element called “liquid crystal panel”, “liquid crystal cell” or the like. The liquid crystal display elements generally include transmission type ones and reflection type ones. The transmission type liquid crystal display element modulates light from a back light provided at the rear side thereof and emits it as transmitted light. On the other hand, the reflection type liquid crystal display element modules incident light and emits it as reflected light. Recently, for a higher definition image display, more compact design and higher light intensity of the projectors, the reflection type liquid crystal display element has been attracting attention as a display device which could implement the higher image definition, more compact design and higher efficiency for light utilization, and is actually commercialized (cf. Japanese Patent Application Laid Open No. 2003-57674).
A conventional reflection type liquid crystal display element will be illustrated and explained here by way of example with reference to FIG. 1. The reflection type liquid crystal display element is generally indicated with a reference numeral 200. As shown, it includes a glass substrate 202 and drive circuit board, provided opposite to each other. The glass substrate 202 has provided thereon a transparent electrode 201 formed from an electrically conductive material such as ITO (indium-tin oxide), and the drive circuit board 204 has provided thereon reflection pixel electrodes 203 formed from an aluminum-based metallic material. A liquid crystal layer 206 is formed by charging a liquid crystal between the glass substrate 202 and drive circuit board 204, sealed at their ends with a sealing member 205. Also, each of the surfaces of the glass substrate 202 and drive circuit board 204, opposite to each other, has provided thereon an alignment layer 207 to align the liquid crystal molecules 206a in a predetermined direction. The drive circuit board 204 is a semiconductor switching drive circuit of the C-MOS (complementary-metal oxide semiconductor) type formed on a silicon substrate. The reflection pixel electrodes 203 formed on the drive circuit board 204 reflect incident light from the glass substrate 202 and applies a voltage to the liquid crystal layer 206.
In the reflection type liquid crystal display element 200, a voltage is applied between the transparent electrode 201 of the glass substrate 202 and reflection pixel electrodes 203 of the drive circuit substrate 204, opposite to each other, thereby applying a voltage to the liquid crystal layer 206. Then, the liquid crystal layer 206 is varied in optical characteristic correspondingly to a potential difference between the electrodes to modulate light passing by the liquid crystal layer 206. Thus, the reflection type liquid crystal display element 200 can assign intensity levels by the light modulation.
The liquid crystals used as such a liquid crystal display element include a horizontally-aligned liquid crystal such as a twisted nematic liquid crystal (will be referred to as “TN liquid crystal” hereunder) whose dielectric anisotropy (a difference Δ∈(=∈(∥)−∈(⊥)) between a dielectric constant ∈(∥) parallel to the long axis of liquid crystal molecules and dielectric constant ∈(⊥) perpendicular to the long axis of the liquid crystal molecules) goes positive. In the TN liquid crystal, when applied with no drive voltage, the liquid crystal molecules are aligned being nearly horizontally twisted in relation to the substrate to provide a display in white in a so-called “normally white display mode”. On the other hand, when applied with a drive voltage, the liquid crystal molecules are erected perpendicularly to the substrate to provide a black level. Also in the TN liquid crystal, since it is necessary to preset a direction in which the liquid crystal molecules are to be erected when applied with a drive voltage, so the liquid crystal molecules are pretilted through about several to 10 deg. in a constant direction in practice.
Also in these days, an liquid crystal display element using a vertically-aligned liquid crystal in which a nematic liquid crystal having a negative dielectric anisotropy is vertically-aligned has been attracting attention for its high contrast and speed of response. In this vertically-aligned liquid crystal, when applied with no drive voltage, the liquid crystal molecules are aligned nearly perpendicularly to the substrate to provide a display in black in a so-called “normally black display mode”. On the other hand, when applied with a drive voltage, the liquid crystal molecules are tilted in a predetermined direction to have the light transmittance thereof varied due to a birefringence developed at that time of tilting.
Also, in the vertically-aligned liquid crystal, since the contrast will not be uniform as shown in FIGS. 2 and 3 unless liquid crystal molecules 206a are tilted in the same direction, so it is necessary to vertically align the liquid crystal molecules 206a by tilting the long axis of the liquid crystal molecules 206a through a slight pretilt angle θ in a constant direction X in relation to a line normal to a drive circuit board 204 having pixel electrodes 203 formed thereon. The pretilting direction X, that is, the direction in which the liquid crystal molecules 206a are aligned, is set nearly diagonal to a device whose transmittance is normally caused to be maximum by a combination with an optical system such as a polarization plate and the like, namely, in a direction of about 45 deg. that is a nearly diagonal direction of the pixel electrodes 203 laid in the form of a nearly square matrix. Also, if the pretilt angle θ is too large, the vertical alignment will be degraded, the black level will rise to lower the contrast and adversely affect the V-T (drive voltage-transmittance) curve. Therefore, the pretilt angle θ is normally controlled to fall within a range of 1 to 5 deg.
The alignment layer which pretilts the vertically-aligned liquid crystal is an obliquely-evaporated layer formed by depositing an inorganic material such as silicon dioxide (SiO2) or the like obliquely onto a substrate or a polymer layer of polyimide or the like having a rubbed surface. The pretilting and pretilt angle are controlled by controlling the direction of incidence and evaporation angle for the obliquely-evaporated layer or by controlling the rubbing direction and conditions for the polymer layer. Normally, the practical pretilt angle is about 45 to 65 deg. in relation to the light normal to the substrate.
Note here that the polyimide layer, which is an organic material, of the alignment layer is deteriorated by light as higher-intensity light is incident upon the polyimide layer, which has recently been a problem to this field of art. On the contrary, being highly stabile against light and not changeable in performance even after the liquid crystal display element has been driven, the polyimide layer which is an organic material is highly reliable for a long period and thus has been attracting attention these days in the field concerned.
Also, the aforementioned reflection type liquid crystal element 200 is normally shipped after covering the surfaces of the reflection pixel electrodes 203 on the silicon substrate with a protective layer of an oxide, nitride or the like to protect the reflection pixel electrodes formed from an aluminum film from being corroded or damaged. Conventionally, a silicon dioxide layer which is easy to form in the LSI process is used as such a protective layer.
FIG. 4 shows the results of measurement of the wavelength dependence of the reflectance of the silicon substrate having the reflection pixel electrodes 203 covered with a 50 nm-thick silicon dioxide layer, and FIG. 5 shows the results of measurement of the wavelength dependence of the reflectance of the silicon substrate after the liquid crystal display element is formed.
Normally, the waveband of light used in the liquid crystal projector is normally on the order of 430 to 700 nm. As shown in FIG. 4, however, the reflectance of the reflection pixel electrodes covered with the silicon dioxide layer is not uniform in the whole waveband but gradually decreases from the blue waveband toward the red one.
Also, the reflectance of the reflection pixel electrodes covered with the silicon dioxide layer is caused by an interference between cell gaps to still undulate as shown in FIG. 5 after the liquid crystal display element is formed but it gradually decreases from the blue waveband toward the red one in the generally same way as in FIG. 4.
That is, the reflectance deterioration in the red waveband is caused by the innate wavelength dependence of the reflectance of the aluminum from which the reflection pixel electrodes are formed.
Also, in the reflection type liquid crystal projector using the aforementioned reflection type liquid crystal element 200, illumination light is emitted from a light source to the reflection type liquid crystal element 200 and the light modulated by the reflection type liquid crystal element 200 is projected by a projection optical system onto a screen on which it will be displayed as an image.
FIG. 6 shows the characteristic curve of the emission spectrum of a high pressure mercury (UHP) lamp used mainly as the light source in the reflection type liquid crystal projector.
As will be known from FIG. 6, the emission spectrum of the UHP lamp is characterized in that the quantity of light is not constant over the waveband but it is decreased in the red waveband. Therefore, in combination with the UHP lamp, the wavelength dependence of the reflectance of the aforementioned reflection pixel electrodes will more reduce the quantity of light in the red waveband.
Thus, in the normal use of the conventional liquid crystal projector, colorization with insufficient red is unavoidable when greater importance is given to the image brightness, which results in declination of the color balance. On the contrary, in case special emphasis is placed on the color balance (white balance, for example), light of green and blue wavelengths has to be attenuated for use correspondingly to light of a red wavelength whose quantity of light is smaller even if the reflectance in the green and blue wavelengths is higher, which results in reduction of the image brightness.